Nanometric composites as improved dielectric structures

ABSTRACT

A dielectric is provided which possesses high dielectric constant and high dielectric strength, while having the ca-pabilities of a polymer. The dielectric comprises a nanometric composite, which includes a stoichiometric nano-particulate filler embedded in a polymer or resin matrix. Filler particles are reduced in physical size to dimension to the same order as the polymer chain length of the host material and interact cooperatively thereby mitigating the associated Maxwell-Wagner process and reducing interfacial polarization. The internal fields for the new formulation are nearly a factor of 10 lower then for conventional (micro) material. The large changes in the internal field of the composite permit engineering of nanocomposite materials with enhanced electric strength and improved voltage endurance properties.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates generally to the field of nanometriccomposites and in particular to a new and useful dielectric structurecomprising nanometric composites.

Electrical insulation is a pervasive technology which is a hugecommercial business ranging from the thin films used in themicroelectronics industry to the large amounts of material used toinsulated high-voltage equipment in the power segment of this market. Inmost instances, the dielectric properties of the insulating structurelimits the design. A 20% improvement in performance would thus havesignificant industrial significance and so the substantial changes thatare indicated by this disclosure are believed to be commerciallyimportant.

Polymers of many types are commonly used as electrical insulation. Theuse of conventional fillers for polymer materials is well known and isusually employed to reduce the cost of a material or to modify one ofthe material properties for a particular application, such as dischargeresistance, thermal expansion, etc. Often the use of such fillers willaffect electrical properties, dielectric strength and loss in a negativeway. In this context, it is thought that fundamental to controlling thedielectric strength of insulating polymers is the cohesive energydensity and the associated free volume of a polymer structure, assuggested in Sabuni H. and Nelson J. K., “Factors determining theelectric strength of polymeric dielectrics”, J. Mats Sci., Vol. 11, p1574, 1976 and Nelson J. K., “Breakdown strength of solids”, EngineeringDielectrics, Vol. 2A, ASTM, 1993. This may be gauged by examining thechanges in electric strength (up to a factor of 10) exhibited by mostpolymers as they are taken through their glass transition temperature.

Nanoparticles are fundamental building blocks in the design and creationof assembled nano-grained larger scale structures with excellentcompositional and interfacial flexibility. However, rather surprisingly,the current push to develop nanomaterials based on nanotechnology hasnot focused much on the opportunities for dielectric materials, butrather centered on optical and mechanical applications, as disclosed inU.S. Pat. Nos. 5,433,906, 5,462,903, 6,344,271, and 6,498,208.

Nonetheless, the few examples in the literature provide encouragementthat this is likely to be fertile ground. Furthermore, some theoreticalreasons for pursuing nanomaterials as a basis for dielectricapplications have been reviewed by Lewis T. J., “NanometricDielectrics”, IEEE Trans on Diel. And Elect. Ins., Vol. 1, pp 812-25,1994 and Frechette M. F. et al., “Introductory remarks onNanoDielectrics”, Ann. Rep. Conf. On Elect. Ins. And Diel. Phen., IEEE,pp 92-99, 2001.

Several patents have also disclosed nanocomposites for alteringelectrical properties. U.S. Pat. No. 6,228,904 discloses a nanocompositestructure comprising a nanostructured filler or carrier intimately mixedwith the matrix, which is preferably polymeric. The nanostructuredfiller can alter certain electrical properties by at least 20%. Thepatent further discloses oxide ceramic nanofiller compositions such asTiO₂ and dielectrics. Nanocomposites with modified internal charge andimproved dielectric strength and voltage endurance are not disclosed.Instead, the focus is on the creation of linear and non-linearconductivity in host materials

U.S. Pat. Nos. 6,554,609 and 6,607,821, which are divisional patents ofthe same parent patent, disclose nano-structured non-equilibrium,non-stoichiometric materials and electrical devices. For example,non-stoichiometric titania in the form of TiO_(1.8) or TiO_(1.3) istaught, as opposed to stoichiometric titania TiO₂. The patents teachthat such nanostructured non-stoichiometric can change the electricalproperties of a material such as electrical conductivity, dielectricconstant, dielectric strength, dielectric loss, and polarization, andare preferred over stoichiometric titania.

U.S. Pat. No. 6,599,631 discloses the use of polymer/inorganic particlecomposites in forming electric and electro-optical devices. However, the'631 patent teaches inorganic nano-particle/polymer composites in whichthe elements of the composite are chemically bonded. Furthermore,although such composites are disclosed as particularly useful for theformation of devices with a selected dielectricconstant/index-of-refraction, the focus of the patent is onelectro-optical properties rather than dielectric properties as relatedto insulation. Appropriate selection of index-of-refraction can beimportant for the preparation of either electrical or optical materials.The index-of-refraction is approximately the square root of thedielectric constant when there is no optical loss, so that theengineering of the index-of-refraction corresponds to the engineering ofthe dielectric-constant. Thus, the index-of-refraction/dielectricconstant is related to both the optical and electrical response of aparticular material. Index-of-refraction engineering can be especiallyadvantageous in the design of optical or electrical interconnects.

The behavior of a typical composite material is often controlled by theproperties of the matrix, the distribution and properties of the filleras well as the nature of their interface. Conventional fillers aremicron size and have been shown to act as foreign bodies, as opposed tocooperative bodies, exerting influence on the resident material viainterfacial properties. In the simplest situation, the bonding of apolymer to a filler can be expected to give a layer of “immobilized”polymer. The size of this layer is critical to the global properties(electrical, mechanical and thermal) of the composite. However, thein-filled material will give rise to space-charge accumulation and anassociated Maxwell-Wagner polarization due to the implanted interfaces.

Furthermore, macroscopic theories of interfacial polarization do notincorporate a molecular approach since the response is given byrelaxation equations if the wavelength is large in comparison withmolecular dimensions. In considering pre-breakdown high-field conductionin pure materials, the existence of localized states within the energyband gap (close to the conduction or valence bands) is usually invoked,giving rise to a mobility edge for electron (or hole) transport. Thesestates are essentially localized on individual molecules. This isbecause, unlike the strong covalent bonds of elemental crystallinesolids, intermolecular binding arises from weak van der Waals' forcesthat do not allow inter-molecular electronic exchange.

A molecular approach is needed for enhancing the dielectric propertiesof insulating structures. Stoichiometric composites are needed in whichfiller particles behave cooperatively with the host matrix rather thanas foreign bodies. Furthermore, composites are needed where space-chargeaccumulation and internal fields are reduced and associatedMaxwell-Wagner polarization due to implanted interfaces is mitigated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a nanometriccomposite in which internal fields are reduced by a factor up to 10 fromconventional composites, and the associated Maxwell-Wagner interfacialpolarization is mitigated.

It is a further object of the present invention to provide a nanometriccomposite in which filler particles behave cooperatively with the matrixof the composite thereby mitigating the associated Maxwell-Wagnerprocess and reducing interfacial polarization.

Accordingly, a nanometric composite is provided for dielectric structureapplications, and comprises nano-particulate fillers embedded in amatrix of polymer or resin. The polymer is essentially any commerciallyavailable polymer.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 are two graphs plotting permittivity and loss tangent as afunction of temperature and frequency for the micro-particulate filledcomposites;

FIG. 2 are two graphs plotting permittivity and loss tangent as afunction of temperature and frequency for the nano-particulate filledcomposites;

FIG. 3 is a graph showing the initial distribution of an electric fieldbased on an electroacoustic study of nano-filled composites;

FIG. 4 is a graph based on the pulsed electroacoustic study of thecomposite with the micron-sized filler;

FIG. 5 is a graph based on the pulsed electroacoustic study of thecomposite with the nano-sized filler;

FIG. 6 is a graph showing charge migration in a 10% microfilled TiO₂sample;

FIG. 7 is a graph showing electroluminescene characteristics in TiO₂composites for base resin, 10% micro filler resin and, 10% nano fillerresin;

FIG. 8 is a graph showing electroluminescence onset field as a functionof TiO₂ loading for the 38 nm sample and the 1.5 μm sample composites;

FIG. 9 a is a graph showing dynamics of electroluminescence in responseto step changes in electric field for the 38 nm TiO₂ sample;

FIG. 9 b is a graph showing dynamics of electroluminescence in responseto step changes in electric field for the 1. 5 μm TiO₂ sample;

FIG. 10 is a graph showing thermally stimulated current spectra for the10% 38 nm TiO₂ sample, and the 10% 1.5 μm TiO₂ sample;

FIG. 11 is a graph showing electric strength of Epoxy/TiO₂ compositesfor the 38 nm filler sample and the 1.5 μm filler sample; and

FIG. 12 is a graph of composite breakdown statistics plotted as aWeibull distribution for the micro filler sample, nano filler sample andbase resin sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A composite dielectric of the present invention possesses highdielectric strength, while having the capabilities of a polymer. Thecomposite also may have high dielectric constant if fillers are chosenwhich have a high dielectric constant. The composite includesstoichiometric nano-particulate filler embedded in a matrix of polymeror resin. The filler particles have a physical size of the same order asthe polymer chain length of the host material and interact cooperativelythereby mitigating the associated Maxwell-Wagner process and reducinginterfacial polarization. The internal fields for the new formulationare nearly a factor of 10 lower then for conventional (micro) material.

The large changes in the internal field of the composite permitengineering of nanocomposite materials with enhanced electric strengthand improved voltage endurance properties. The composition and physicalconfiguration of the dielectric can be designed to specific applicationrequirements such as high voltage insulation or electrical fieldgrading.

In a preferred embodiment of the present invention, the compositeincludes 10% inorganic oxide in the form of Titanium Dioxide (TiO₂)filler particulates with nano dimensions embedded in a Bisphenol-A epoxy(Vantico CY1300+HY956) polymer. Bisphenol-A epoxy is a preferred polymerbecause it is benign (i.e. without other fillers or dilutents), it has alow initial viscosity, and has a glass transition below 100° C.

However, the invention is not limited to titanium dioxide as filler andcan include a broad range of inorganic oxides, metal oxides, titanates,silicas, particles coated with coupling agents such as silanes andtriblock copolymers, and even nano-sized polymers. Silica-based fillersin particular are suitable due to their low loss characteristics. Theavailability of nanoparticles of a wide range of inorganic oxides offersthe possibility of creating a range of new materials with tailoredproperties and benefits (e.g. variation in relative permittivity andlinearity).

Still, the invention is not limited to the broad range of filler groupswhich have been disclosed since they are only mentioned as examples inorder to enable one to practice the invention. In practice, one ofordinary skill in the art will understand that a variety of differentfillers can be used based on the application that is desired. Aluminumoxide may be preferred because it is inexpensive or zinc oxide may beused because of its non-linear nature.

If a dielectric with high dielectric constant is desired, nanocompositeswith fillers having high dielectric constant may be used such asTitanium Dioxide. Conventional filler materials include the oxides ofaluminum, zinc, and titanium. Aluminum oxide has a linear current versusvoltage relationship and is widely in use. Conversely, zinc oxide ishighly non-linear. Titanium dioxide however, is an attractive materialdue to its inherently high dielectric constant of 90-100 versus aluminumoxide and zinc oxide both of which are in the 6 to 7 range.

The invention is also not limited to Bisphenol-A matrix polymer. Thematrix polymer may be a thermoplastic or a thermoset polymer. Othersuitable polymers include other variants of epoxy, polyolefins such aslow density polyethylene (LDPE), cross-linked polyethylene, andpolypropylene. Polypropylene in particular is economically inexpensiveand typically used in the capacitor industry. Additionally, the matrixpolymer may include ethylene propylene rubber, functionalized polymerssuch as polyetherimide, and essentially any other commercially availablepolymer, provided that the filler is available in nano-particulate size.

As applied to the formation of dielectric structures, Example 1 belowwill illustrate that 10% appears to be the optimum loading % of fillerfor the configurations tested. Measurements have been made up to 40%,but at that level the mechanical properties are degraded to the pointthat the material is of little use in such applications. However if thenanophase particles are polymeric, such as for example nano-particulatepolyurethane, then 40% loading % of filler may be suitable. For oxides,the loading % ranges between about 2 and about 20% for suitability withdielectric applications.

Example 1 below demonstrates through a variety of studies thatsignificant interfacial polarization associated with conventionalfillers, is mitigated in the case of particulates of nanometric size.The studies include Differential Scanning Calorimetry (DSC),Photoluminescence measurements, Dielectric Spectroscopy, Space ChargeAssessment via a Pulsed Electro-Acoustic (PEA) apparatus,Electroluminescence, Thermally Stimulated Currents, and ElectricalStrength Measurements.

EXAMPLE 1

Composites were provided for micro-particulates and nano-particulates ofTitanium Dioxide embedded in a resin matrix of Bisphenol-A epoxy. A listof the composites is shown below in Table 1.

Test samples of the composites were formed by molding between polishedsurfaces, held apart by spacers, as described in Griseri V., “Theeffects of high electric fields on an epoxy resin”, Ph.D. Thesis,University of Leicester, 2000. The molded films range in thicknessbetween 500 and 750 μm. The weighed resin and hardener were degassed at35° C. and the relevant dried particulate fill was incorporated into theresin by mechanical stirring. Due to their small size, surfaceinteractions for nanoparticles, such as hydrogen bonding, becomemagnified. This means that the particles tend to agglomerate anddispersion in resins is quite difficult, even in polymers that should berelatively compatible. Hence, in the case of nano-particles, large shearforces are needed in the mixing process to obviate unwanted clusteringof the particles. For most electrical characterization, the cast filmwas provided with evaporated 100 nm aluminum electrodes.

Differential Scanning Calorimetry (DSC)

A Stanton Redcroft DSC 1500 calorimeter was used to thermallycharacterize the materials. Results on the determination of glasstransition temperatures are provided in Table 1 below for post-curedsamples from which it is evident that the nano-material reduces T_(g) incontrast to the larger size particles that have the opposite effect.This suggests that particles of nanometric dimensions behave in asimilar way to infiltered plasticizers, rather than as “foreign”materials creating a macroscopic interface. TABLE 1 Material + FillerSize (nm) Loading (%) Tg(° C.) CY1300 Resin N/A N/A 63.8 CY1300 + TiO₂Micro (1500) 1 76.1 CY1300 + TiO₂ Micro (1500) 10 73.9 CY1300 + TiO₂Micro (1500) 50 79.9 CY1300 + TiO₂ Nano (38) 1 62.9 CY1300 + TiO₂ Nano(38) 10 52.4 CYl300 + TiO₂ Nano (38) 50 62.1

Photoluminescence Laminar molded specimens using both micro- andnano-particulates were subjected to photoluminescence measurements asdepicted in Table 2 below. for excitation wavelengths from 280 to 360nm. The shift in the peak wavelength in the presence of thenanoparticles (6^(th) column in Table 2) implies that the emittingspecies have had their environment altered. On the assumption that theemission is excimeric in origin, this suggests that the nanoparticlesmay cause minor conformational changes sufficient to bind the excimerunits more tightly. The magnitude of the peak emission in thenano-composite case is also behaving in an entirely different way(decreasing with increasing excitation wavelength) when compared withthe response of the conventional micron-sized filler. TABLE 2 Ex. λ BaseResin 10% Micro 10% Nano (nm) Pk λ Pk Mag Pk λ Pk Mag Pk λ Pk Mag 280413 7.0 411 25.5 418 29.6 320 409 21.5 411 35.8 420 44.6 340 407 65.4405 128.1 412 35.2 360 408 85.8 406 151.2 423 14.1

Dielectric Spectroscopy Some insight into the way that the incorporationof materials on nanometric dimensions affect the dielectric propertiesmay be obtained by examining the variation of the real and imaginarycomponents of relative permittivity as a function of temperature andfrequency, wherein the temperatures from bottom to top are 293 K, 318 K,343 K, 368 K, and 393 K. This has been done for the TiO₂ material usinga Solartron H. F. frequency response analyzer (Type 1255) in combinationwith a Solatron Dielectric Interface, Type 1296.

Examples for the micro- and nano-filled materials are shown in FIGS. 1and 2 respectively. At a nominal 10% (weight percent) particulateloading, the spectra of the resin when filled with particles of micronsize (1.5 μm) are virtually indistinguishable from the base resin. Thissuggests that the low frequency process is probably associated withcharges at the electrodes and not due to particulates in the bulk.

With the filler replaced with 10% of nanometric size TiO₂ (38 nm averagediameter measured by TEM), the main differences seen relate to a markedmodification of the process seen in the base resin at low frequenciesand high temperatures. For the nanometric material the process exhibitsa flat tan δ response at low frequencies in marked contrast to themicron-sized filler. This suggests that a percolation conduction processis operative. In the presence of the nano-filler, the mid frequencydispersion is noticeably reduced.

The nano materials are clearly inhibiting motion (see PEA resultsbelow). The mid-frequency process shows a small change in estimatedactivation energy from 1.7 eV to 1.4 eV. The magnitude of this processis reduced in the case of nanoparticles since the side chainsresponsible for the mid-frequency dispersion bind to the particlesurface.

Reduction of the particulate loading from 10 to 1% (by weight) did nothave any very obvious fundamental changes, but the nano-filled materialthen does start to exhibit a low frequency response more typical of thebase resin and micro-filled material, suggesting that changes engineeredby the nanomaterials do require loadings greater than a few percent.

Space Charge Assessment

In order to determine whether nanomaterials function cooperatively asopposed to providing sites for interfacial polarization, a PulseElectroAcoustic (PEA) study has also been conducted to assess the fielddistortions in the bulk. The method has been described in Alison J., “AHigh Field Pulsed Electro-Acoustic Apparatus for Space Charge andExternal Circuit Current Measurement within Solid Dielectrics”, Meas.Sci Technol., Vol. 9, pp 1737-50, 1998.

FIGS. 3, 4, and 5 show the results of the electroacoustic study. Thefigures are labeled with Voltage, V (kV), charge, ρ (C.m⁻³), andelectric field, E (kV.mm⁻¹). The double headed arrow indicates the 726micron thickness of the sample.

The laminar samples were subjected to direct voltages. According to FIG.3, the initial distribution of stress shows little deviation from thenominal 4.3 kVmm⁻¹ uniform level across the bulk. However,characteristic results are shown in FIGS. 4 and 5 for the micro- andnano-materials (10% loading) respectively after several hours ofstressing. These plots show the charge, potential and fielddistributions, for a 3 kV steady DC field applied. The 1.5 μm fillergenerates substantial internal charge, in marked contrast to thenano-material which behaves in a similar way to the base resin.

FIG. 4 shows several distinctive features including (a) heterochargeaccumulation of both signs leading to steep internal charge gradients;(b) a cathode field augmented to over 40 kVmm⁻¹ (10× the nominal value);and (c) field reversal yielding a point of zero stress which willgreatly complicate charge transport.

Transient PEA studies permit the establishment and decay of chargeprofiles to be viewed in time.

Measurements, such as that depicted in FIG. 6 for a step voltageapplication of 3 kV on a 10% micro-filled specimen, indicate thatincreases in the size of the charge peaks occurs over a 4 hour periodwith little macroscopic change to the complex internal distribution. Thestable stationary positioning of these peaks may be due to theinteraction of space charge with local polarization to create aself-compensating situation.

However, there are very substantial differences in the time constantsassociated with the migration and decay of charge for the micro-andnano-composites as is illustrated below in Table 3 in comparison withoptical electroluminescence emission. In contrast to the micro-filledmaterial, the decay of charge in the nano-filled TiO₂ is very rapid withinsignificant homocharge remaining after just 2 minutes. Although thereis some injection of negative charge at the cathode, the nano-filledmaterial is characterized by much less transport perhaps brought aboutby the larger density of shallower traps. TABLE 3 38 nm TiO₂ 1.5 μm TiO₂Charge Decay (s) 22 1800 Light Decay (s) <60 1200

Electroluminescence

The light emission from a ˜4 μm point molded into the resin samples isdepicted in FIG. 7 for a 10% loading. The curves 100, 110, and 120rexspectively represent the base resin sample, the 10% micro fillerresin sample, and the 10% nano filler resin sample. The pre-dischargeelectroluminescence is measured with a 13-dynode EMI 9789Bphotomultiplier tube having a bialkali spectral response connected inscintillation counting mode (i.e. the light is determined by countingpulses during a fixed interval, usually 60 s). Two hours was allowed forthe photocathode to stabilize before measurements were attempted. Thefield, E, in FIG. 7 is that calculated at the individual tip based on J.H. Mason, “Breakdown of solids in divergent fields” Proc. IEE Vol. 102C,1955, pp 254-63: $\begin{matrix}{E = \frac{2V}{r\quad{\ln\left( {4{d/r}} \right)}}} & (1)\end{matrix}$where r is the tip radius and d the inter-electrode gap.

While the level of activity for the nanomaterial is generally somewhatless, the salient feature is the light onset level. The nanomaterialrequires 400 kVmm⁻¹ to register output above the background countwhereas both the base resin and the micromaterial start emitting atstresses which are only half that value -about 180 kVmm⁻¹. This compareswith the 178 kV mm⁻¹ found by V. Griseri et al. “Electroluminescenceexcitation mechanisms in an epoxy resin under divergent and uniformfield” Trans IEEE, Vol. DEI-9, 2002, pp 150-60, using uniform fields ina similar resin system. However, this comparison may be fortuitous sincethe previous study speculated that the emission is the result of abipolar charge recombination mechanism. In this divergent field case, itis more likely that the light results from the downward transition ofexcited species formed by electron injection in the high tip field. Whenthe electroluminescence output is examined as a function of loading(FIG. 8), it is clear that enhancement in the onset is again a maximumat about 10% as is indicated below in the section entitled ElectricStrength. In FIG. 8, electroluminescence onset field is plotted as afunction of sample loading for the 38 nm sample and 1.5 μm sampleplotted respectively as curves 150 and 160.

Electroluminescence measurements have also been made as a function oftime to observe the way in which the materials react to a step change instress of 600 kVmm⁻¹. FIGS. 9 a and 9 b depict the dynamics of lightemission for 10% nano- and micro-filled materials respectively. The timeresponse of the base resin is of the same form as shown in FIG. 9 a forthe nanocomposite. Comparison of these under both switch-on andswitch-off transients indicate that the two materials respond verydifferently as will be discussed at greater length later. However, it isalso important to recognize that light is emitted for a period after theapplied field is removed, strongly suggesting that it is the Poisson andnot the Laplacian field that is intimately involved withelectroluminescence.

Thermally Stimulated Currents

Laminar samples of both micro- and nano-filled resin were subjected tothermally stimulated discharge having been poled at 115° C. at a stressof 55 kvcm⁻¹. The temperature ramp rate was 0.05° Cs⁻¹. Typical plotsfor the two different types of material are shown in FIG. 10, wherecurve 180 represents 10% 38 nm TiO₂ and curve 190 represents 10% 1.5 umTiO₂ fillers.

The glass transition temperature, T_(g) for the base resin is 89° C.,and Differential Scanning Calorimetery measurements have alreadydemonstrated that T_(g) can be expected to change slightly with the TiO₂filler size for this resin. Accordingly, the TSC peaks at about 90° C.may be associated the main chain relaxation (the α-peak). Similarly, thepeak at about 70° C. can be associated with the β-relaxation. However,the characteristics above 100° C. are very different indeed for the twofiller sizes. This region, designated as the ρ peak shown in FIG. 10, isdue to the release of space charge in epoxy resins as identified by AKawamoto et al., “Effects of interface on electrical conduction in epoxyresin composites”, Proc. 3^(rd) Int. conf. on Prop. & App. of Diel.Mats., IEEE, 1991, pp 619-22.

Electrical Strength Measurements

Short-term electric strength measurements have been measured under DCconditions with a ramp rate of 500 Vs⁻¹. FIG. 11 depicts the meanbreakdown gradient (for a population of 10 samples) for the base resin,as well as the micro- and nano-composites as a function of fillerloading (% by weight). Curve 200 represents the 38 nm sample and curve210 represents the 1.5 μm sample. The advantage in electric strengthattributable to the nano-sized filler is clear, and an optimum loadingof about 10% is indicated. Although, for high loadings (close to thepercolation limit), the advantages are eroded, and the degradation inmechanical properties makes such very high loadings unattractive.

In summary, very marked differences in charge accumulation are seen infilled materials depending on whether the filler has micron ornanometric dimensions.

Not only does the incorporation of nanoparticles yield a dielectricstrength close to that of the base polymer, but FIG. 11 alsodemonstrates that the β parameter (dispersion) is unchanged by theaddition, in contrast to the microfilled material where a significantchange of slope in the Weibull plot in FIG. 12 is indicated. FIG. 12shows a graph of composite breakdown statistics plotted as a Weibulldistribution where line 300 represents micro filler resin, line 310represents nano filler resin, and line 320 represents base resin. Whilethe presumed reduction of free volume on the substitution ofnanoparticles may be instrumental in improving the electric strength asdisclosed by Kawamoto et al. above, the results presented here alsostrongly suggest that the improvements in electric strength may belinked to the control of the internal charge within the bulk.

The electroluminescence onset studies reported here suggest that thelarge surface area inherent nanoparticles has created a mechanism forelectron scattering which will skew the energy distribution withbeneficial results; i.e. a higher voltage is required for light onset.However, in seeking reasons for the marked differences seen in manyaspects of behavior when nanoparticulates are incorporated, FIGS. 9 aand 9 b would seem to be pivotal. The escalation of light inmicro-filled resin over a period of about an hour following energizationsuggests that the tip field is augmented by the establishment ofheterocharge (positive) in front of the point cathode. Indeed, theemission of light following the removal of the applied stress dictatesthat the tip field is sustained by charge in the bulk.

Careful examination of the PEA results indicates that such a region ofcharge is, indeed, formed when the infilled material is of large (μm)dimension. In contrast, the nanomaterial exhibits the maximumelectroluminescence on switch-on, indicating that any charge whichaccumulated acts to shield the point electrode and reduce the high-fieldlight emission. This effect will also be incorporated in FIG. 7 sincethere was sufficient time allowed for charge modification to take place.Although not shown in FIG. 7, cases were documented where the onset oflight occurred measurably earlier for the micro-filled material than forthe base resin.

The PEA method also allows the decay of charge to be estimated followingthe removal of the applied field. Table 3 above provides estimates ofthe decay time constants obtained from the decay of the electrode imagecharges in a PEA experiment for TiO₂ nano- and micro-filled epoxy incomparison with electroluminescence decay. While the absolute numbersare not comparable because of the differing geometries, nevertheless,the very substantial differences brought about by the filler size aredemonstrated by both techniques and, again, points to the effects ofinternal fields.

Charges trapped at the interfaces formed by the microparticles will beneutralized by charges of opposite sign conveyed to the interfaces byohmic conduction giving rise to a TSC transient. This means that thenature of the TSC peak (and even its polarity) will depend on both therelative permittivity and the conductivity of the constituent materials.Following the work of J. van Turnhout in “Electrets”, Chapter 3,Springer-Verlag, 1980, (Topics in Applied Physics, Vol. 33 ed. G. M.Seessler), the TSC transient due to the annihilation of charge, σ, isgiven by: $\begin{matrix}{{i(t)} = {\frac{\mathbb{d}\sigma}{\mathbb{d}t}\frac{\left\lbrack {{ɛ/ɛ_{1}} - {{g(T)}/{g_{1}(T)}}} \right\rbrack}{\left\lbrack {{s/s_{1}} + {{g(T)}/{g_{1}(T)}}} \right\rbrack\left( {1 + {ɛ\quad{s_{1}/e_{1}}s}} \right)}}} & (2)\end{matrix}$where the ratios of the permittivities ε/ε₁ and the conductivities g/g₁will determine the polarity of the discharge current during the TSCthermal ramp.

Consequently, for microfilled TiO₂, a negative Maxwell-Wagner peak issometimes experienced, particulary at low poling temperatures. However,the poling temperature used in FIG. 10 (115° C.) is above T_(g) and thusthe ρ-peak should be fully developed as disclosed by J. van Turnhout,and the position of the peak is independent of the poling conditions ashas been found in this study. The significant finding here is that thenano-composite does not exhibit the marked ρ-peak characteristic ofMaxwell-Wagner interfacial effects in the conventional material.

The PEA results taken in conjunction with the Dielectric Spectroscopyand DSC studies suggest that significant interfacial polarization isimplied for conventional fillers which is mitigated in the case ofparticulates of nanometric size, where a short-range highly immobilizedlayer develops near the surface of the nanofiller (1-2 nm). This boundlayer, however, influences a much larger region surrounding the particlein which conformational behavior and chain kinetics are significantlyaltered. This interaction zone is responsible for the material propertymodifications especially as the curvature of the particles approachesthe chain conformation length of the polymer.

Evidence suggests that the local chain conformation and configurationplay major roles in determining the interactions of a polymer withnanofillers, as is evidenced here by the DSC results of Table 1. Thepolymer binding to the nanoparticles replaces some of the cross-linkingand thus loosens the structure. In contrast, the micron scale caseproduces significant Maxwell-Wagner polarization giving rise to thecharacteristics of FIG. 4.

In the case of nanofillers, there is evidence that a grafted layer isformed by the absorption of endfunctionalised polymers onto the surfaceespecially when the functional groups are distributed uniformly alongthe polymer backbone. Hence the local chain conformation is critical todetermining the way in which bonding takes place (and thus the cohesiveenergy density). The defective nature of nanoscale particles can beexpected to enhance the bonding if chemical coupling agents (CVDcoatings on nanoparticles or triblock copolymers) are employed.

The present invention has a variety of applications. For example, interms of volume, one of the most significant applications of the presentinvention is in the field of power generators and motor insulation.Epoxy mica, which is discharge resistant, currently has an insulativelife of approximately 10 years and is ideal for the field of powergenerators and motor insulation. Pacemakers are another suitableapplication of the present invention because the present inventionallows insulator suppliers/manufacturers to increase voltage and reducesize of insulative materials since less material is required.

Finally, it is anticipated that the use of smaller molecules assynthetic additives, chemical coupling agents, triblock copolymers, etc.may permit an element of self assembly of these structures, and create aclass of “smart” materials based on nanocomposites to provide autostress relief and other forms of self compensation. It may be possibleto self-assemble nanodielectrics by providing chemical structures with“hooks” which provide preferential attachment points for thenanostructured materials allowing automatic and predictable selfassembly.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A nanometric composite for use in dielectric structures to reduceinterfacial polarization, comprising: a matrix of polymer; andnano-particulate fillers; wherein internal charge is modified.
 2. Ananometric composite according to claim 1, wherein the polymer isselected from the group consisting of epoxy, polyolefin, ethylenepropylene rubber and polyetherimide.
 3. A nanometric composite accordingto claim 1, wherein the filler is selected from the group consisting ofinorganic oxides, metal oxides, titanates, silicas, particles coatedwith coupling agents, and nano-sized polymers.
 4. A nanometric compositeaccording to claim 1, wherein particulate size is comparable to polymerchain length so that the particulate and the matrix polymer interactcooperatively.
 5. A nanometric composite according to claim 1, whereinthe composite has a filler loading of 10%.
 6. A nanometric composite foruse in dielectric structures to reduce interfacial polarization,comprising: a matrix of thermoset polymer; and nano-particulate fillers;wherein particulate size is comparable to polymer chain length so thatthe particulate and the matrix polymer interact cooperatively so thatinternal charge is modified.
 7. A nanometric composite according toclaim 6, wherein the polymer is selected from the group consisting ofepoxy, polyolefin, ethylene propylene rubber and polyetherimide.
 8. Ananometric composite according to claim 6, wherein the filler isselected from the group consisting of inorganic oxides, metal oxides,titanates, silicas, particles coated with coupling agents, andnano-sized polymers.
 9. A nanometric composite according to claim 6,wherein the composite has a filler loading of 10%.
 10. A dielectricstructure comprising a nanometric composite comprising: a matrix ofpolymer; and nano- particulate fillers; wherein internal charge ismodified.
 11. A dielectric structure according to claim 10, wherein thepolymer is selected from the group consisting of epoxy, polyolefin,ethylene propylene rubber and polyetherimide.
 12. A dielectric structureaccording to claim 10, wherein the filler is selected from the groupconsisting of inorganic oxides, metal oxides, titanates, silicas,particles coated with coupling agents, and nano-sized polymers.
 13. Adielectric structure according to claim 10, wherein particulate size iscomparable to polymer chain length so that the particulate and thematrix polymer interact cooperatively.
 14. A dielectric structureaccording to claim 10, wherein the composite has a filler loading ofabout 2% to about 20%.
 15. A dielectric structure according to claim 10,wherein the composite has a filler loading of about 10%.
 16. Adielectric structure according to claim 12, wherein the compositecomprising a nano-size polymer has a filler loading ranging from about2% to about 40%.